Submicrometer dimensional measurement is essential for semiconductor processing. It is necessary to measure the width of a critical line structure, for example, the gate width of a transistor. More importantly, critical line-width measurements are often used to characterize VLSI processing steps. For example, in photolithography, the measured width is directly related to critical process parameters such as focus distances and exposure periods.
Traditional techniques based on thresholding of the intensity of an optical image taken with a standard optical microscope are not accurate enough for submicrometer measurements. By using a confocal or correlation microscope, good results for narrow trenches down to widths of 0.4 .mu.m can be obtained. However, for photoresist lines and arrays of lines and trenches, the relation between measured line-width and nominal line-width is not always linear. Furthermore, in the latter case, it is sometimes difficult to pick out the width of lines less than 0.7 .mu.m wide.
For critical dimensional measurements, one is usually interested in line-widths. Thus, instead of acquiring a whole two-dimensional image, one only needs to acquire a line-scan at the same x location on the object every time its focus position changes. FIG. 1 schematically shows a microscope objective 11 focused at a spot 12 with respect to an object 13 which includes a line structure 14 to be scanned. The object is placed on a stage 16 which can be moved in the x-y-z directions so that the focal spot impinges upon different surfaces and locations on the object. To obtain a linescan, the object is moved along a line in the x-direction so there is scanning in the x-directions and then stepped in the z-direction to scan another line at another elevation. The image acquired by the microscope is applied to a CCD camera 17 which digitizes the signals which are stored in a computer 18. The computer controls the positioning stage. In the preferred embodiment, the focal spot extends over an area to create an image of a plurality of lines of the line structure and the output of the CCD camera is lens scanned to give readings for the x-direction. The stage is only moved to observe a different area. In either embodiment, the signals can then be processed and displayed on display 19 as a cloud plot. The cloud plot is a gray scale display of intensity as a function of horizontal position x and axial focusing position z along a line. FIGS. 2A and 2B are examples of cloud plots of 0.5 .mu.m wide dense resist structures recorded by a confocal microscope and a correlation microscope (the Mirau Correlation Microscope), respectively.
The traditional approach to line-width measurements is to plot the intensity of the cloud plot as a function of x at various focus planes, z, of interest. For example, as illustrated in FIGS. 3A and 3B, for a Mirau correlation microscope and confocal microscope, respectively, L.sub.top is the top width of the photoresist strips, L.sub.bot is the width at the trench bottoms and L.sub.resist measures the width of the resist strips at the substrate interface. We shall call the respective line scans (along x) at the top of the resist; the bottom of the trench and the bottom of the resist l.sub.top (X), l.sub.bot (x), and l.sub.resist (x), respectively. Typically, the width of the actual line is then taken to be the width between the 1/2 or 1/3 power points of linescans taken in the x direction at these positions in z. For the confocal microscope, the intensity linescan is used; while for the correlation microscope, the linescan can be taken at the transformed intensity image. FIG. 2C is the transformed intensity image of the cloud plot of FIG. 2B; it is equivalent to the intensity cloud plot for the confocal microscope. Typically, it is standard practice to take the width of the line to be the distance between the 1/2 or 1/3 power points of a linescan in the x direction at a fixed position z corresponding to either the top or bottom of a trench or line. It is difficult to determine in the cloud plot the exact axial locations where the linescans l.sub.top (x), l.sub.bot (x), and l.sub.resist (x) should be taken, as the absolute locations may shift due to optical waveguiding or resonance effects inside the line structures. The shapes of the lines l.sub.top (x), l.sub.bot (x), or l.sub.resist (x) may change as the true line-widths vary. Typically these regions are picked out by looking for the position of maximum intensity in a scan in the z direction, as illustrated in FIGS. 3A and 3B. Consequently, it is not easy to construct an automatic measurement algorithm which tracks the true hne-width consistently.